1. Field of the Invention
This invention relates to the photolithography technology, and more particularly relates to an exposure process to define in a photoresist layer a plurality of first patterns having a certain pitch and a wider second pattern between them without suffering from lens aberrations, and to a photomask set used in the exposure process.
2. Description of the Related Art
As the linewidth of integrated circuit (IC) process is unceasingly reduced, the accuracies in the positions and the critical dimension of a pattern/space are more and more important. The accuracy in the critical dimension of a pattern/space depends on the depth of focus, lens aberrations and so forth, wherein the lens aberrations include coma aberration that is a type of aberration where light rays from a point on the reticle do not converge to a single point in the image.
For example, when a plurality of line patterns having a pitch of about 325 nm and a wider line pattern of about 430 nm in width between them are to be defined in a photoresist layer using a single photomask having corresponding patterns in presence of coma aberration, the variation in the critical dimension of the spaces beside the wider line pattern defined in the photoresist layer may be up to about 7.5%. In the prior art, the variation can merely be reduced to about 5.5% by adding an assistant space-type feature at the middle of the wider line pattern on the photomask.